The invention relates to a method of reducing artefacts in reconstruction images of a reconstructed object image and an outer region which surrounds the object image obtained by means of computer tomography. The reconstruction images are obtained by scanning a layer of an object by means of radiation which penetrates the object (for example, X-radiation) so that the object, as well as the region surrounding the object, is irradiated in a large number of directions by radiation beams which are situated in the layer. The radiation is detected by detectors which produce measuring data which characterizes the absorption of a radiation beam along a measuring path. The measuring data is used to derive image data which represent the absorption values at elements of an uncorrected reconstruction image of the irradiated layer. The image data in the reconstruction image associated with a measuring path is then used to derive error signals to correct the uncorrected reconstruction image to produce a corrected reconstruction image. The invention furthermore relates to a device for performing the method.
U.S. Pat. No. 3,936,636 describes an iterative reconstruction/correction method where the image data in reconstruction images approaches the values of the actual absorption coefficients in the successive iteration steps. To this end, a layer is irradiated in different directions by parallel radiation beams so that measuring data are determined which characterize the attenuation of radiation traversing different measuring paths. The data is converted into image data by a transformation, said image data being a first approximation of the radiation absorption coefficients of elements of a reconstruction image. The reconstruction image then consists of square elements of a matrix. By way of a transformation which is inverse to the first transformation, the image data is converted into fictitious measuring data which, if the image data is correct, corresponds to the original measuring data. A comparison between the measuring data and the fictitious measuring data produces correction data which is subsequently converted, by way of said transformation process, into correction image data which is a measure of the deviation of the image data stored by each element from the actual absorption coefficients. The modification of the image data by the correction image data results in a reduced deviation. When a given number of correction calculations is performed, the image data are close to the actual absorption coefficients.
The inverse transformation for producing the fictitious measuring data is performed so that the image data of the elements associated with the corresponding measuring path are summed along each measuring path which extends in the reconstruction image and which corresponds to the geometry of the backprojection, or along path of the radiation passing through the object.
Because the measuring paths which intersect the elements at a finite angle have a given width which is determined by the width of the detectors, the image data of each element are multiplied by a weighting factor which is determined by the distance between the center of the square element and the center line of the measuring path.
The described method enables a reduction of the deviations of the image data from measured absorption coefficients which arise during the reconstruction by means of said transformation process, the fictitious measuring data being compared with the original measuring data. However, a reduction of image artefacts caused by measuring errors such as "aliasing" and quantum noise (as described by G. Kowalski and W. Wagner in OPTICA ACTA, 1977, Vol. 24, No. 4, pages 327 to 348) cannot be achieved by means of the described method.